Tunnel effect in ferromagnetic half-metal Co$_2$CrAl-superconductor heterostructures

TL;DR

This paper investigates tunnel effects in ferromagnetic Co$_2$CrAl-superconductor heterostructures, revealing enhanced differential conductivity due to spin-polarized electron accumulation, and estimates near-perfect spin polarization in Co$_2$CrAl films.

Contribution

It demonstrates a novel method to estimate spin polarization in ferromagnetic films using tunnel junctions with superconductors, showing near 100% polarization in Co$_2$CrAl.

Findings

Normalized differential conductivity exceeds that of normal metal-superconductor junctions.

Spin polarization degree P in Co$_2$CrAl is close to 1 at 4.2 K.

Enhanced conductivity attributed to spin-polarized electron accumulation.

Abstract

Ferromagnetic half-metal Co$_2$CrAl films and tunnel contacts Co$_2$CrAl - insulator (I) - Pb are fabricated and investigated. It is found that the normalized differential conductivity $\sigma ^{\rm FS} $ of such tunnel junctions with low resistance is larger than the normalized differential conductivity $\sigma ^{\rm NS} $ of known normal metal - I - superconductor type tunnel junctions. It is shown that the observed increase in $\sigma ^{\rm FS} $ is caused by the accumulation of spin polarized electrons in a superconductor and can be used for estimating the spin polarization degree $P$ in ferromagnets. This method shows that $P$ of L2$_1$-type ordered Co$_2$CrAl Heusler alloy films at $T = 4.2{\rm K}$ is close to 1.